XWIKI allows pages to contain invalid characters () but bails out on history

Similiar to issue XWIKI-8995 I've got problems accessing historical versions of a page. The current version displays fine.

Analysis (e.g. catalina.out) shows errors like

• ''Exception while reading version [xxx.x] for document id [x,x,x,x,x,x]"
• "Caused by: com.xpn.xwiki.XWikiException: Error number 2002 in 2: Error parsing xml"
• "Caused by: org.dom4j.DocumentException: Error on line 255 of document : Character reference "&#1" is an invalid XML character. Nested exception: Character reference "&#1" is an invalid XML character."

Analysis shows, that for some unknown reason an invalid character (
, viewable on edit) has been entered (may be some cut and paste-error) and saved rendering this and all versions after it unable to properly display in history (regardless of difference or page-view).

Thus I file a bug as saving invalid contend should be disallowed. If the contend were to be allowed it should display properly in history, too.

I fixed the issue within the database with a statement like
update xwikircs set xwr_patch=replace(xwr_patch, "
", "") where xwr_patch like '%&#1%';